Finite Element Analysis of Failure Modes of Buried City Gas Pipeline Tees

Literature Overview

The paper by Liu Ke and Shao Dawei, published in Contemporary Chemical Industry in 2014 (Vol. 43, No. 2, pp. 274–276), presents a plastic finite element analysis of failure modes of buried city gas pipeline tees under combined internal pressure and external out-of-plane bending moment loading. The authors are affiliated with the Shenyang Special Equipment Inspection and Research Institute. The study was funded by the General Administration of Quality Supervision, Inspection and Quarantine of China under the public welfare industry research project on buried pipeline leakage accident analysis and emergency response key technology research (Project No. 201010025). The research employs ANSYS software with both geometric and material nonlinearity to analyze the plastic failure modes of tees with and without external wall local thinning defects.

Technical Background

City gas distribution pipelines are critical infrastructure components that supply natural gas to residential, commercial, and industrial consumers. These pipelines are typically buried underground and are subject to various loading conditions, including:

• Internal gas pressure (operating and maximum allowable)
• External loads from soil pressure and traffic
• Thermal expansion and contraction
• Seismic loads
• Corrosion-induced wall thinning

Tee fittings are used at branch connections in gas distribution networks to connect lateral lines to the main pipeline. The geometry of a tee creates complex stress states, particularly at the junction of the branch and the run, where stress concentrations develop. The combination of internal pressure and external bending moments can lead to plastic deformation and, ultimately, failure if the loads exceed the structural capacity.

Failure Modes of Tees

The study identifies and analyzes several failure modes for tees under combined loading:

Finite Element Modeling

Model Configuration

The FEA models were configured to represent tees with the following characteristics:

• Geometry: Standard tee geometry with specified run and branch diameters and wall thicknesses
• Material: Carbon steel with elastic-plastic constitutive model (bilinear hardening)
• Mesh: Shell elements with refined mesh at the branch-run junction and defect locations
• Boundary conditions: Simulating the connection to adjacent pipe segments
• Loading: Combined internal pressure and out-of-plane bending moment

Defect Modeling

The study considers local external wall thinning defects at three locations on the tee run:

1. Bottom of the run (6 o'clock position): Subject to maximum compressive stress under downward bending moment
2. Belly of the run (3 or 9 o'clock position): Subject to shear stress and moderate hoop stress
3. Shoulder of the run (near the branch junction): Subject to high stress concentration from the branch connection

Each defect location represents a different stress state and therefore a different failure mechanism. The defect is modeled as a localized reduction in wall thickness with specified dimensions (length, width, and depth of thinning).

Results and Key Findings

Stress Distribution Analysis

The FEA results provide detailed stress distribution maps for each configuration. Key observations include:

• The branch-run junction is the location of maximum stress concentration in the intact tee, with stress levels significantly exceeding the nominal hoop stress.
• The addition of an external wall thinning defect significantly increases the local stress at the defect location, particularly when the defect is located at the bottom of the run under bending.
• The failure mode transitions from uniform plastic yielding (in the intact tee) to localized plastic collapse (in the defective tee) as the defect size increases.

Failure Mode Prediction

The study predicts the following failure mode sequence as loads increase:

1. Elastic regime: Linear stress-strain behavior, no plastic deformation.
2. Initial yielding: Localized yielding at the branch-run junction or defect location.
3. Plastic spreading: Plastic zone expands from the initial yield location.
4. Limit state: Gross plastic deformation, unbounded displacement, structural failure.

Effect of Defect Location

The location of the external wall thinning defect has a significant effect on the failure load:

These relative failure load values are approximate and depend on the specific defect dimensions and tee geometry.

Engineering Practice Implications

Design and Assessment Guidelines

The FEA results provide important guidelines for the design and fitness-for-service assessment of buried gas pipeline tees:

1. Defect location matters: The same defect size at different locations produces significantly different failure loads. Bottom-of-run defects are the most critical under typical buried pipeline loading conditions.
2. Combined loading is critical: The interaction between internal pressure and external bending moment must be considered in the assessment. A tee that is adequate for pressure alone may fail under combined loading.
3. Defect size thresholds: There exist critical defect sizes beyond which the failure load drops below acceptable safety limits. These thresholds should be established for each specific tee geometry and material grade.
4. Branch junction integrity: The branch-run junction remains the most critical location even in the presence of external defects, and local reinforcement or design optimization at this location can significantly improve the overall structural capacity.

Inspection and Monitoring Recommendations

Based on the failure mode analysis, the following inspection priorities are recommended for buried gas pipeline tees:

Comparison with Standards

The FEA results should be compared with the assessment methods provided in relevant standards:

The FEA approach used in this study aligns with the methodology described in API 579/ASME FFS-1 Part 5, which allows for detailed FEA-based assessment of specific components with complex geometries and combined loading.

Key Questions and Reflections

Several aspects of this study merit further discussion. First, the study focuses on external wall thinning defects, but internal corrosion can also be a significant concern in gas pipelines, particularly where moisture ingress occurs through defective coatings or at weld joints. Second, the study considers only static loading, but fatigue loading from pressure cycling and traffic-induced bending should also be considered for long-term integrity assessment. Third, the study does not address the effect of material properties on the failure behavior, such as the difference between low-carbon steel (e.g., L245/L360) and higher-grade steels (e.g., L485/L555). Fourth, the study does not consider the effect of soil-structure interaction, which can significantly influence the external loading on buried tees.

Study Insights

This paper provides a rigorous FEA-based analysis of tee failure modes under realistic buried pipeline loading conditions. The systematic investigation of defect location effects is particularly valuable, as it provides engineers with clear guidance on which locations are most critical for inspection and assessment. The consideration of combined internal pressure and external bending moment loading reflects the actual service conditions of buried gas pipelines, making the results directly applicable to integrity management programs. The study also highlights the importance of tees as critical components in gas distribution networks—they have complex geometries that create high stress concentrations and are therefore more vulnerable to corrosion damage than straight pipe sections. Engineers responsible for gas pipeline integrity should give special attention to tee fittings in their inspection and assessment programs, particularly at locations where external wall thinning is detected at the bottom of the run or near the branch junction. The FEA methodology demonstrated in this study provides a powerful tool for making quantitative fitness-for-service decisions, but it should always be supplemented with appropriate safety factors and validated against experimental data where available.